clocksource: timer-imx-gpt: remove include of <asm/mach/time.h>

The include of <asm/mach/time.h> is not needed at all, and causes build
error in some cases.  Remove it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>